A grieving mom plans to sue Metro-North and the city for $40 million, charging their failure to fix a broken fence led her 14-year-old son to drown in the treacherous waters of the Bronx River.

Hannah Johnson has filed a notice of claim against the railroad, which owns property near where her son, Joseph Johnson, died. He slipped through a fence, boarded a makeshift raft and wound up in the water – even though he didn’t know how to swim.

The notice of claim – the first step toward filing a lawsuit – joins an earlier one Johnson filed against the city. The city also owns land near the spot that gave Joseph and his pals access to the river, family lawyer Richard Altman said.

He said he plans to seek $20 million in damages for Joseph’s wrongful death and more than $20 million for the teen’s pain and suffering.

Altman said he’s still investigating who actually owns the fence. It should have been kept in good repair to protect the public, particularly children, from entering the area, he said.

Johnson drowned in the Bronx River after he and his friends boarded a makeshift raft on June 21. Although he couldn’t swim, other boys challenged him to jump in the water.

Family members have insisted Joseph was afraid of water, and suggested he was pushed. But cops have called the drowning death an accident. The boy’s mother could not be reached.

Spokespersons for the railroad and the city declined comment because of the pending litigation.